Up to now, the presence of elevated intracranial pressure (ICP) in acute neurological disorders is suspected by clinical and neuroimaging findings, but its verification depends on invasive techniques. Based on our experimental findings of rapid dilatation of human optic nerve sheaths (ONS), we investigated whether this phenomenon not only happens under chronic, but also under acute conditions of intracranial hypertension. Using optic nerve sonography the ONS was measured at 3 mm behind the papilla in axial transbulbar view. Thirty-nine children admitted to the intensive care unit (ICU) were examined. Of these 24 were being treated for elevated ICP (head trauma, metabolic disorder) and were compared to control patients (outpatients). The ONS diameter (ONSD) found in ICU patients with elevated ICP ranged up to 6.8 mm and was significantly enlarged compared with normal data (Wilcoxon's test, P = 0.007). The ONSD of ICU patients without pressure elevation was in the same range as that of control patients (2.7-4.0 mm). Considering the error of measurement (0.35 mm), the ONSD is regarded as definitely enlarged when 5 mm is exceeded in children above age 4. In younger children, smaller ONSD have to be taken into consideration. We conclude that ultrasound studies of the optic nerve may contribute information about the acutely increased ICP in critically ill patients.